The present invention relates to optical scanning apparatus and a method for manufacturing cathode ray tubes and, more particularly, to optical scanning apparatus in which the shape and size of exposed photosensitive material on the faceplate of a cathode ray tube may be accurately controlled.
The exposure of this photosensitive material provides a means for delineating the pattern of other material applied to the faceplate for generating, filtering or blocking light or for other functions. In a typical method, a phosphor is dusted onto the surface of the photosensitive material, after which the material is selectively exposed. Then, the unexposed photosensitive material is removed from the faceplate by wellknown techniques. An important step in this method is the act of exposing the photosensitive material at the proper location on the faceplate.
Non-scanning methods for exposure of photosensitive material on the inner surface of a faceplate of a cathode ray tube are known. In one method, the photoresist, such as dichromated polyvinyl alcohol, is exposed by light from an ultraviolet light source, the light passing through an aperture mask registered with the faceplate. The ultraviolet source is a mercury arc lamp whose output is concentrated to pass through a small source aperture and then dispersed to fully illuminate the aperture mask. The proper intensity distribution, which is not necessarily uniform, across the aperture mask is obtained by controlling the intensity distribution at the source aperture and by the insertion of a graded neutral density filter between the source aperture and the aperture mask.
For proper registration of the phosphor pattern on the faceplate with the electron beam landings in the assembled tube, the light rays from the ultraviolet source during photoresist exposure should parallel the electron beam trajectories as they pass through the various apertures in the aperture mask. Due to aberrations in the magnetic deflection process, the apparent location of the electron beam source varies with the deflection angle. Thus, a fixed optical point source alone cannot simulate the deflected electron source over the entire faceplate. To introduce the necessary off-axis correction factors into the optical exposure system, a special aspheric lens is inserted into the system between the light source and aperture mask, with a separate lens being required for each of the three electron gun positions. The contour of each lens is designed such that the light source as seen through the lens from each point on the faceplate has the correct lateral location in the source plane to produce rays passing through the aperture mask with the same angle of incidence as an electron beam through the same aperture in an assembled tube. Design calculations for these lenses are difficult and costly, especially as maximum deflection angles become larger.
In most cases, when a tube design is modified by changing the maximum deflection angle, deflection yoke winding pattern or position, faceplate curvature, aperture mask spacing, or certain other parameters, a new lens set and graded neutral density filters are needed. Optimizing the new design may require a trial-and-error procedure which could involve the fabrication of additional lenses and filters.
Various scanning exposure systems are also known. In such a system, a small light beam is scanned over the aperture mask so as to expose the photosensitive material adjacent to the light-transmitting regions or apertures in the mask. For example, a scanning exposure system is described in the British Patent Specification No 1,257,933. In this patent, a scanned laser beam is used in conjunction with an aperture mask and photosensitive material for delineating phosphor patterns on faceplates for color CRTs. However, this patent does not provide for correction of the inherent discrepancy between elecetron beam landings and phosphor locations.
Another scanning exposure system is described in the U.S. Pat. to Geenen et al., No. 3,876,425. In this system, the effective light beam source is actually translated about a source plane to provide correlation between phosphor locations and electron beam landing locations. Such beam translation eliminates the need for the aspheric lens which is necessary in the non-scanning exposure system. In the system described in the Geenen patent, the effective light beam source is the center of a mirror which deflects the beam toward the faceplate. An optical system always insures that the beam from the actual source is always directed to the center of the scanning mirror. The scanning mirror is carried by an assembly that rotates the mirror about two orthogonal axes to provide scanning and that translates the mirror along two orthogonal axes to provide movement of the center of the mirror in the source plane. The optical system includes a plurality of mirrors, bearing assemblies and a telescoping member.
This system has limitations which render it less than suitable for use in a production environment for cathode ray tubes. First, a scanning exposure system must have accurate optical alignment characteristics, i.e., the ability to repeatedly position the light beam at a predetermined point on the faceplate. The mechanical and optical system described is of such a complicated nature that it is doubtful that such alignment characteristics could be obtained. More specifically, the large number of rotating parts and simultaneously rotating and translating parts of the system could result in misalignment with continued use as is necessary in a production environment. Furthermore, the feature of actually translating the effective light beam source in a plane adds complexity to the electronic system which is necessary to control the scanning and mirror translation functions. More specifically, each time the mirror is translated, the beam, if not corrected by the scanning function, would impinge upon other than the desired faceplate location. Thus, the translating and scanning functions are interdependent.